SWIFT's expensive, yes, and that would suck. I'd just funnel it via Investec - they can open a HSBC account in the UK for their clients. Haven't had the need thus far but something I've been looking at to aid in moving funds offshore.
Make sure you register with IB's UK branch then. UK bank account to IB is pretty fast...like 2-3 hrs and costs nothing.
The HSBC thing sounds very odd though. Investec has UK accounts - and opens them for people planning on moving to the UK. Bit surprised that they'd facilitate opening accounts with a bank they're not associated with AFAIK. If you can get an investec uk one do that assuming you're fine with the £10 fee...I've got both and not a huge fan of the hsbc one for various reasons
You'd be SWIFTing either way though (even investec za to investec uk) unless you use an internet thing. I use xendpay.com for smaller transfers which seems good but haven't trusted it with big transfers yet